A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system, and some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens, that stem from one allele (or very closely linked genes), collectively form a blood group system.

Blood types are inherited and represent contributions from both parents. A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT).

Many pregnant women carry a fetus with a different blood type from their own, and the mother can form antibodies against fetal RBCs. Sometimes these maternal antibodies are IgG, a small immunoglobulin, which can cross the placenta and cause hemolysis of fetal RBCs, which in turn can lead to hemolytic disease of the newborn, an illness of low fetal blood counts which ranges from mild to severe.

Serology

If an individual is exposed to a blood group antigen that is not recognized as self, the immune system will produce antibodies that can specifically bind to that particular blood group antigen, and an immunological memory against that antigen is formed. The individual will have become sensitized to that blood group antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other tissue cells), often leading to destruction of the cells by recruitment of other components of the immune system. When IgM antibodies bind to the transfused cells, the transfused cells can clump. It is vital that compatible blood is selected for transfusions and that compatible tissue is selected for organ transplantation. Transfusion reactions involving minor antigens or weak antibodies may lead to minor problems. However, more serious incompatibilities can lead to a more vigorous immune response with massive RBC destruction, low blood pressure, and even death.

ABO and Rh blood grouping

Anti-A and Anti-B, the common IgM antibodies to the RBC surface antigens of the ABO blood group system, are sometimes described as being "naturally occurring"; however, this is a misnomer, because these antibodies are formed in infancy by sensitisation in the same way as other antibodies. The theory that explains how these antibodies are developed states that antigens similar to the A and B antigens occur in nature, including in food, plants, and bacteria. After birth an infant gut becomes colonized with normal flora that express these A-like and B-like antigens, causing the immune system to make antibodies to those antigens that the red cells do not possess. So, people who are blood type A will have Anti-B antibodies, blood type B will have Anti-A antibodies, blood type O will have both Anti-A and Anti-B antibodies, and blood type AB will have neither. Because of these so called "naturally occurring" and expected antibodies, it is important to correctly determine a patient's blood type prior to transfusion of any blood component. These naturally occurring antibodies are of the IgM class, which have the capability of agglutinating (clumping) and damaging red cells within the blood vessels, possibly leading to death. It is not necessary to determine any other blood groups because almost all other red-cell antibodies can develop only through active immunization, which can occur only through either previous blood transfusion or pregnancy. A test called the Antibody Screen is always performed on patients who may require red-blood-cell transfusion, and this test will detect most clinically significant red-cell antibodies.